At precisely 5:12 am on the morning of April 18, 1906, the northernmost 296 miles of the San Andreas Fault jolted many Californians from their sleep. The initial quake and catastrophic fires that followed are estimated to have sent nearly 3,000 to an early grave. Geologists claim this temblor to have been one of the most significant earthquakes of all time. This claim holds true when one considers that we are still learning from this 107-year-old quake.

Residents of northern California, and more specifically the San Francisco Bay area where the earthquake’s epicenter was closely located to, had only approximately 20-25 seconds warning from the initial foreshock before the massive quake shook the region violently for nearly a full minute. The full force of the quake was felt from southern Oregon down to Los Angeles and as far inland as central Nevada. Early findings from this great quake still influence zoning rules as they apply to habitable buildings and their proximity to the fault trace.

Just to the south of San Francisco, in San Mateo County, is the quaint town of Portola Valley, known for their early embrace of the field of geology. As well as being the first town in California to be the subject of a geologic map, it was also were the first municipality in the state to have its own resident geologist on the city payroll. This was due, in no small part, to the fact that the section of the San Andreas Fault responsible for the 1906 quake just happens to run right through their town.

However, even with the great attention that was paid to the quake and Portola Valley’s commitment to all things geologic, the exact location of the fault trace has been a mystery over the past century. But with work being performed by local and regional researchers, northern California will no longer be living a lie. The study employed a combination of new technologies along with old photographs to help locate the exact location of the fault line.

“The story of this fault trace reminds me of that game [called “Telephone”] we all played as kids, where one person whispers a sentence to somebody and they whisper it to the next person and by the time it goes around the circle, the sentence has completely changed,” says Ted Sayre, a geologist at Cotton, Shires and Associates, a geotechnical consulting firm in Los Gatos that serves as the “town geologist” for Portola Valley.

“The initial trace reported in 1906 was fairly accurate, but the story kept changing over the years until 100 years later, the maps and reports were no longer true to what actually happened,” says Sayre, co-author of the new study in the Bulletin of the Seismological Society of America.

The team’s work began when they noted discrepancies in the hand-drawn maps originally created in the aftermath of the earthquake. They also noted many of the photographic negatives from the day had been accidentally transposed during printing leading to a reverse view of the fault line area. Each mistake, cited originally in a 1908 report, had been cited in at least eight subsequent studies, propagating the incorrect information for some time. This led to the fault trace being off by as much as 100 meters from its actual locations.

Adding to the difficulties in mapping the fault lines is the heavy brush that is native to Portola Valley. This covering is very effective at obscuring both the ground surface and the fault trace. “Portola Valley is covered with very dense chaparral and thick masses of poison oak,” says Chester Wrucke, a retired US Geological Survey geologist and lead author of the current study. “Geologists are a hardy bunch, but even they don’t want to wade around in poison oak,” stated Wrucke, a resident of Portola Valley.

Assisting with the study was Wrucke’s son, Robert. The younger Wrucke is an independent computer engineer and is also based in Portola Valley. His expertise in the study was employed when he utilized bare-earth lidar technology. This technology is able to peer through vegetation, allowing the team to accurately map the area. Once these measurements were completed, the team overlaid the lidar imaging over the old photos, helping them to pinpoint the exact locations where the early reports identified erroneous fault traces.

“The new technology is really neat, but it was really the archives that provided the best clues,” says the elder Wrucke. The team cited a specific photo taken in 1906 to highlight the superiority of their method in correcting an ancient wrong. In the photo, a well-dressed couple posed directly next to a surface scar. Noting the man’s waistcoat buttons were on the correct side of his coat – the right side – helped them to confirm this photo was not one of the many that had been flipped in processing. Other photos clearly showed skyline vantages and road curves that would otherwise be impossible. This, they say, was a clear indicator the negative had been reversed.

This study, which required several years of research, has been able to determine, say Sayre and the Wruckes, the exact location of the fault, which runs through Portola Valley along a single trace.

Mary Lou Zoback, a seismologist at Stanford University who was unaffiliated with the study, claims this more accurate location for the fault is more important than settling an age-old curiosity. California, prone to an elevated level of seismic activity, has established strict zoning codes, especially within a certain proximity to active faults. “Determining the location of the active trace of major faults is very important, especially in terms of zoning.”

The zoning codes Zoback refers to were established in the 1972 Alquist-Priolo Earthquake Fault Zoning Act, a response to the San Fernando earthquake that rocked southern California that year. Alquist-Priolo was passed in order to lessen property and life loss with quakes that presented a significant surface rupture. Specifically, Alquist-Priolo forbids new houses and buildings meant for human habitation to be built within 15 meters of a known active fault trace.

Zoback continues, “In terms of the shaking hazard, shifting the fault a few hundred meters doesn’t really make much difference. The implications really center around the Alquist-Priolo Act.” She concludes, “In a residential area, drawing [15-meter] boundaries surrounding the fault can block out a lot of area.”